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Old 10th November 2009, 21:00   #91
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Originally Posted by high_octane View Post
To sum up ...
Torque : related more to how MUCH energy can be produced in the engine(which could be increased by using a high octane fuel and so on)
RPM : related more to how smoothly the pistons rotate overcoming friction due to weight,cylinder interior surface (along which the piston moves) which is dependent on the engine design

Please correct if I am wrong anywhere !
You've missed one big factor. The rpm can be increased easily by reducing the compression ratio - which will reduce the torque. Petrol engines normally rev higher as the petrol burns easily with lesser compression. Infact you can't compress petrol beyond a point without it exploding (and that's why they reduce the combustibility by additives - high octane fuels).

Low compression makes it easier to rev higher. What you get is a linear power delivery. The initial torque is moderate but as the rpm goes up, the torque still stays healthy and with a good gear ratios, very high top speed can be achieved.

On the contrary, diesel engines require higher compression to burn and hence the compression ratio is very high (normally in the range of 18.5:1 to 20:1). This produces high initial torque, but due high compression the engine cannot maintain the torque at high rpm. The peak torque is delivered at a narrow range. Gear ratios become all the more crucial to effectively utilise this hugh surge in torque coming in at a low rpm.
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Old 11th November 2009, 10:12   #92
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Originally Posted by oss View Post
This produces high initial torque, but due high compression the engine cannot maintain the torque at high rpm.
Cannot get this point. Can you expalin a bit here.
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Old 11th November 2009, 12:56   #93
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shan2nu and r@CYR@y, I kind of get what you guys are trying to say. you are saying, the increase in the rpm can be controlled by the design of the engine only. That means, the rpm depends on how smoothly the pistons move and hence affect how faster the transmission rotates. This is like a big doubt solved !
So the increase in the rpm levels in order to increase the power of the car (at a fixed speed,remember) cannot be accomplished if the design of the engine is not altered. One way,however, could be by making the same car light (using lighter materials like lighter wheels etc ). On the contrary ,I think the torque of the car (ie the pulling power) can be increased easily isnt it ?

To sum up ...
Torque : related more to how MUCH energy can be produced in the engine(which could be increased by using a high octane fuel and so on)
RPM : related more to how smoothly the pistons rotate overcoming friction due to weight,cylinder interior surface (along which the piston moves) which is dependent on the engine design
Quote:
You've missed one big factor. The rpm can be increased easily by reducing the compression ratio - which will reduce the torque. Petrol engines normally rev higher as the petrol burns easily with lesser compression. Infact you can't compress petrol beyond a point without it exploding (and that's why they reduce the combustibility by additives - high octane fuels).

Low compression makes it easier to rev higher. What you get is a linear power delivery. The initial torque is moderate but as the rpm goes up, the torque still stays healthy and with a good gear ratios, very high top speed can be achieved.

On the contrary, diesel engines require higher compression to burn and hence the compression ratio is very high (normally in the range of 18.5:1 to 20:1). This produces high initial torque, but due high compression the engine cannot maintain the torque at high rpm. The peak torque is delivered at a narrow range. Gear ratios become all the more crucial to effectively utilise this hugh surge in torque coming in at a low rpm.
You guys are making it more confusing. Torque is nothing but rotational force applied to an object about its axis. You use torque to open a gate, turn on the tap, tighten wheel nuts etc...

How much torque is created depends on 2 things. One is the perpendicular force applied to the object and the other is the distance from where that force is applied.

The further you are from the axis point, lesser the force required to create torque. You can try this with your gate. First hold the end of the gate and push it, it'l move without any probs, then slowly start moving closer towards the hinges of the gate. The closer you get to the hinges, more the effort required to push it.

In the same way the combustion inside the engine generates energy that pushes the piston down but the torque produced is also dependant on the distance from the axis which is decided by the lenght of the stroke.

Lower compression ratio doesn't mean more revvs. Performance engines run high compression ratio and still produce their torque and power at very high rpms. The reason for diesel engines revving low is due to the distance the piston has to travel to complete each cycle due to the longer stroke.

Shan2nu

Last edited by Shan2nu : 11th November 2009 at 12:59.
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Old 11th November 2009, 14:09   #94
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Originally Posted by high_octane View Post

I thought its the other way ??? I mean rpm should control the horsepower and not the other way around isnt it ?
Rpm definitely controls the horsepower, higher the rpm higher will be the power, the best example is a f1 car, a 3.0 NA V10 produces around 900+ Bhp @ around 18000rpm, you see here power output is 300Bhp/litre, without higher rpm these figures are impossible (even if you use turbos or superchargers).

Now to be very simple lets see an example below..
let car A has 100Bhp and 10Nm of torque where as car B has 10Bhp and 100Nm torque, when these two cars run beside each other, can you tell what is going to happen?

The car B will take the lead and accelerate faster than car A because of the torque while the car A will be struggling to accelerate, after some time when car A gains momentum it will overtake car B (Which was ahead of it) using its power, Its this simple..
If you want faster accelerating car then MORE TORQUE should be advantageous
If you want top speed, go for big BHP numbers

Please note that increasing bhp also increases the torque indirectly but increasing the torque will increase the BHP by a marginal amount only..

Keep revving..
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Old 11th November 2009, 14:30   #95
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Quote:
Originally Posted by high_octane View Post
The increase in the rpm can be controlled by the design of the engine only. That means, the rpm depends on how smoothly the pistons move and hence affect how faster the transmission rotates.
.
.
.
Please correct if I am wrong anywhere !
Increasing the rpm is not a simple thing, as rpm means revolutions per min, some of our indian cars can go upto 6500rpm easily that means
6500/60 ~ 103 revolutions per second, Can you imagine what will be the situation in the engine,
one rev = one movement of piston upwards +
one movement of piston downwards +
rotation of camshaft +
opening and closing of intake valves +
opening and closing of exhaust valves +
firing of the spark plug +
addition of fuel rich air and many more


Imagine all these steps 100 times in a second you will get all your doubts clarified..

F1 cars have 300revs/sec utilizing the worlds most sophisticated and latest technology and remember those engines last only for 1500 kms each..

Keep revving..
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Old 11th November 2009, 14:49   #96
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Originally Posted by r@CYR@y View Post
...
This is a very simple example you have taken where you have considered the peak power and torque values, which by the way happen at different RPMs.

It is impossible to predict the outcome of A vs B in the example above without knowing the power/torque curve and the gear mated to the transmission.
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Old 11th November 2009, 15:03   #97
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Rpm definitely controls the horsepower, higher the rpm higher will be the power, the best example is a f1 car, a 3.0 NA V10 produces around 900+ Bhp @ around 18000rpm, you see here power output is 300Bhp/litre, without higher rpm these figures are impossible (even if you use turbos or superchargers).
BHP is relative to engine rpm only in NA form. With forced induction you can get much more torque and power at lower rpms.

Rocam's B16 Turbo has been tested at 350whp@9200rpm. Even if we consider 15% loss, thats a good 402bhp at the crank. Thats around 250bhp/ltr being produced at 9200rpm.

So basically this personally owned engine is producing 83% of an F1 cars specific output at 51% revvs.

I wonder what those 600+bhp Civics must be putting out, And im not even considering all those mosnter drag engines!!!

NA and FI just cannot be compared.

Shan2nu

Last edited by Shan2nu : 11th November 2009 at 15:08.
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Old 11th November 2009, 15:06   #98
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Originally Posted by watashi75 View Post
It is impossible to predict the outcome of A vs B in the example above without knowing the power/torque curve and the gear mated to the transmission.
Well, I forgot to add there that both cars are similar except the power and torque, I think this would be sufficient for a simple explanation..

Keep revving..
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Old 11th November 2009, 15:18   #99
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Quote:
Originally Posted by Shan2nu View Post
BHP is relative to engine rpm only in NA form. With forced induction you can get much more torque and power at lower rpms.
Shan2nu
Yes you are right, but even in FI cars power and torque delivery varies with rpm all the time, turbo kicks in only after engine reaches certain rpm isnt it () and again the because of the huge pressure inside the cylinders, the engine needs careful and safe design..
we cannot get everything as we want and the way we want

Keep revving..
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Old 11th November 2009, 15:50   #100
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Yes you are right, but even in FI cars power and torque delivery varies with rpm all the time, turbo kicks in only after engine reaches certain rpm isnt it () and again the because of the huge pressure inside the cylinders, the engine needs careful and safe design..
I never said it didn't. My post had nothing to do with power/torque curves or turbo lag.

I just said that you dont always need high rpm to get power/torque from an FI engine. An FI engine doesn't need to revv all the way to 18,000rpm to produce 300bhp/ltr.

So high rpm doesn't always mean higher specific output. You can have engines producing more specific output at lower revvs with FI.

Shan2nu
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Old 11th November 2009, 15:53   #101
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Just to remind you have a look below..

Quote:
Originally Posted by Shan2nu View Post
BHP is relative to engine rpm only in NA form.
Shan2nu
Keep revving..
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Old 11th November 2009, 17:00   #102
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Remind me what?

I didn't say BHP is relative to engine rpm.

I said BHP its relative to engine rpm only in "NA" form.

Which is what i've been trying to explain. Just bcoz a NA engine needs 18,000 rpm to produce 300bhp/ltr, it doesn't mean FI engines need to revv that high.

If someone tells me his NA engine produces 100bhp/ltr@6000rpm, i wont believe him.

But if someone says his FI engine does 100bhp/ltr@6000rpm, i wont write it off unless i have a look at the specs sheet.

Moreover, just bcoz one engine has higher rpm doesn't always mean it has more power. Simple example is OHC VTEC which does 106bhp@6800rpm vs ANHC which does 116bhp@6600rpm. Small variations in output are bound to happen.

Shan2nu

Last edited by Shan2nu : 11th November 2009 at 17:13.
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Old 11th November 2009, 17:48   #103
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Quote:
Originally Posted by Shan2nu View Post
But if someone says his FI engine does 100bhp/ltr@6000rpm, i wont write it off unless i have a look at the specs sheet.
Shan2nu
Cool down a little bit,
I think you didn't get what I am trying to say or maybe I didn't explain it properly ,

And the fact is

Evolution X makes 145BHP/ltr @ 6800rpm

and it is a FI
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Old 11th November 2009, 18:11   #104
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@Shan2nu
glad you pointed out about the distance part( I had'nt thought about it). so is the distance from the axis to the point where force is applied always kept at maximum in all vehicles so that the torque developed due to the distance part atleast is always maximum ?
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Old 11th November 2009, 20:18   #105
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I think you didn't get what I am trying to say or maybe I didn't explain it properly ,
Ok then, tell me what exactly you meant to say in this para.

Rpm definitely controls the horsepower, higher the rpm higher will be the power, the best example is a f1 car, a 3.0 NA V10 produces around 900+ Bhp @ around 18000rpm, you see here power output is 300Bhp/litre, without higher rpm these figures are impossible (even if you use turbos or superchargers).


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